The USGS annual survey found the latest three-year average for the otter population dropped to 2,711, a 3.6-percent drop from last year’s three-year average. The number of otter pups dropped by 11 percent.

Scientists say the decline could be linked to a number of human and natural factors, including heavy storms, infectious disease and shark attacks.

”There are a lot of arrows pointing towards elevated pollution from the land,” said Tim Tinker, a research biologist with the USGS. “That includes chemical pollutants, toxins, increased use in fertilizers and also something we call bio-pollution, which is disease-causing parasites from the land that are getting into the ocean through runoff, and causing death in the sea otter population.”